<!DOCTYPE html>
<meta charset="utf-8">
<span>る</span>
<div>X</div>
<div>X</div>
<script>
document.body.offsetHeight;
const span = document.querySelector('span');
const divs = Array.prototype.slice.call(document.querySelectorAll('div'));
for (const e of divs) {
  const n = span.cloneNode(span);
  e.appendChild(n);
  e.offsetHeight;
}
</script>
